The present invention relates to an apparatus and method for controlling exhaust gas recirculation (EGR) in an engine, and more particularly, to controlling of an EGR valve that adjusts the EGR amount.
Japanese Laid-Open Patent Publication Nos. 55-148950, 10-238412, and 2000-337172 describe vehicle engines incorporating exhaust gas recirculation (EGR) apparatuses. An EGR apparatus includes a recirculation passage, which connects an exhaust passage of the engine to an intake passage of the engine, and an EGR valve, which is arranged in the recirculation passage. When the EGR valve is opened, the difference between the pressures of the upstream side and downstream side of the EGR valve sends exhaust gas from the exhaust passage to the intake passage through the recirculation passage. The exhaust gas in the intake passage lowers the combustion temperature in the combustion chambers of the engine and suppresses the production of nitrogen oxides (NOx).
Japanese-Laid-Open Patent Publication No. 8-270454 describes a vehicle engine incorporating a variable geometry turbocharger and an EGR apparatus. The variable geometry turbocharger includes a turbine, which is provided with variable vanes, and is arranged in the exhaust passage. The opening degree of the variable vanes are varied to produce supercharged pressure even at low engine speeds. Thus, the engine produces high power from low speed ranges.
The EGR amount is determined from the EGR valve opening degree and the difference between the pressures at the upstream side and downstream side of the EGR valve. An electronic control unit (ECU) installed in a vehicle obtains the fuel injection amount based on the operating state of the engine and obtains a target opening degree of the EGR valve corresponding to the fuel injection amount from a predetermined formula used for feedback control. The ECU feedback controls the EGR valve so that the opening degree of the EGR valve becomes equal to the target opening degree. The formula is set under the assumption that the difference between the pressures at the upstream side and downstream side of the EGR valve is normal. That is, the EGR valve target opening degree, which is obtained through the formula, achieves the optimal EGR amount when the pressure difference is in a normal state.
Further, the ECU determines whether to perform EGR in accordance with the operating state of the engine. When determining to perform EGR, the ECU actuates the EGR valve from a fully closed state to the target opening degree.
For example, in a diesel engine incorporating a variable geometry turbocharger, if the depressed acceleration pedal is released when the engine is in a high speed high load state and then immediately depressed again to accelerate the engine speed, the difference between the exhaust pressure and the intake pressure temporarily becomes greater than the normal value. If the EGR valve is opened in this state, a greater amount of exhaust gas is recirculated to the intake passage in comparison with when the difference between the exhaust pressure and the intake pressure is in a normal state. This releases fumes from the exhaust passage.
More specifically, when the depressed acceleration pedal is released in a state in which the engine speed is high, fuel injection is stopped. Further, the EGR valve and the throttle valve are closed. In this state, the variable vanes of the variable geometry turbocharger are controlled to increase the opening degree. However, the movement of the variable vanes, which are driven through feedback control, is delayed from the change in the fuel injection amount, which is controlled in accordance with the depressed amount of the acceleration pedal, the actuation of the throttle valve, and the actuation of the EGR valve, which is controlled in accordance with the fuel injection amount. Thus, in a short period from when the acceleration pedal is depressed, the EGR valve is closed but the variable vanes are virtually not moved in the opening direction.
The exhaust pressure in the exhaust passage increases in a state in which the EGR valve is closed and the variable vanes are not moved in the opening direction. The intake pressure in the intake passage decreases immediately after the depressed acceleration pedal is released since the throttle valve moves in its closing direction when inertia continues to operate the engine in a high speed state. This produces an excessive difference between the exhaust pressure at the upstream side of the closed EGR valve and the intake pressure at the downstream side of the closed EGR valve.
When the depressed acceleration pedal is released and then depressed again within a short period of time, the variable vanes are maintained at a relatively small opening degree until the intake pressure of the intake passage, or the supercharged pressure, reaches its target value. As the rotating speed of the turbine increases and the intake pressure reaches its target value, the variable vanes move in the opening direction. Due to inertia, a certain period of time is required for the rotating speed of the turbine to increase. Thus, the variable vanes do not move immediately in the opening direction even if the acceleration pedal is depressed again. Accordingly, if the EGR valve is opened from a fully closed state when the acceleration pedal is depressed again, the excessive difference between the exhaust pressure and the intake pressure causes excessive recirculation of the exhaust gas to the intake passage. This discharges fumes from the exhaust passage.
Such events are prominent in diesel engines incorporating variable geometry turbochargers. However, even in a diesel engine incorporating a fixed geometry turbocharger or a diesel engine that does not incorporate a turbocharger, when the acceleration pedal is manipulated in a manner that suddenly changes the engine operating state, the excessive difference between the exhaust pressure and the intake pressure may produce fumes.